The production of gas (particularly nitrogen) by membranes is considerably developed in recent years, everywhere in the world, complementarily to the conventional production by cryogenic means, because it has the following advantages:
Excellent supply reliability: PA1 Low production cost; PA1 The possibility of providing, at very attractive costs, according to the applications in question, of gases of suitable purity. PA1 the case in which it is necessary to produce high purity gases at a high flow rate (for example less than 0.1% residual impurities), while being competitive relative to the cryogenic production method or the preferential absorption method; PA1 as to the production of CO or H.sub.2, when they are obtained by recovery from materials rejected from industrial sites (the characteristics particularly of composition and flow rate of the entering mixture to be treated being in a restrictive way fixed by the industrial site producing the mixture to be treated): to produce with a good technico-economic compromise the purity of CO or of hydrogen required by the final utilization station; PA1 in the case of the production of hydrogen from certain entering mixtures from refineries, whose heavy hydrocarbon concentration is high, and for which according to the hydrogen purity sought at the inlet, the permeate side extraction of a large portion of the hydrogen of the mixture, or even a portion of the light hydrocarbons, gives rise to an enrichment of the gas obtained on the residue side of the membrane in "heavy" constituents. This phenomenon leads to an elevation of the dew point of the residual gas relative to the dew point of the entering mixture and can thus give rise, according to the temperature of the operation of the membrane, to a hydrocarbon condensation, which is extremely deleterious to the polymer fibers which constitute the membrane. PA1 supplying with a good technico-economic compromise, a high purity gas (for example nitrogen), PA1 supplying with a good technico-economic compromise, and from an entering mixture whose characteristics are fixed by the industrial site producing this entering mixture, a gas flow pure for example as to hydrogen or CO to the extent of the needs of the final utilization station, PA1 in the case of the separation of hydrogen from mixtures rich in heavy hydrocarbons, effecting this separation under good output conditions, with reduced risks of untimely condensation, which is damaging to the membrane. PA1 is characterized in that two separators are used whose selectivities as to said gas are different.
The principle is that, under the influence of a difference of partial pressure on opposite sides of the membrane, there is obtained on the permeate side a low pressure mixture enriched in the most permeable components and at the outlet of the membrane (also called the "residue" or "reject" side), a mixture at a pressure near the supply pressure (of the entering mixture) and which is enriched in the less permeable components.
There are thus used to produce nitrogen (often termed "impure") from air, semi-permeable membranes having good properties for the separation of oxygen from nitrogen (selectivity), for example of the polyimide type, the mixture enriched in oxygen being obtained on the permeate side. These membranes are often termed "nitrogen" membranes.
As to the production of hydrogen or CO, there is most often conducted a recovery from mixtures from certain industries, which are separated on semi-permeable membranes having good properties for the separation of hydrogen relative to other components of the mixture, for example of the polyaramide type, the mixture enriched in hydrogen being obtained on the permeate side, the mixture enriched as the case may be in hydrocarbons or in CO being obtained from the residue side of the membrane. These membranes are often called "hydrogen" membranes.
It is evident that the performances obtained will depend very largely on the conditions of use of the membrane, such as the temperature, the pressure of supply to the membrane, or the content of the supply mixture in the component which it is desired to remove from the permeate side.
As to temperature, it is also known that when increasing the temperature of operation of the membrane, most often the permeability and hence the productivity of the membrane will increase, but its selectivity (for example O.sub.2 /N.sub.2) and hence the efficiency declines. Most often, the expression "temperature of operation of the membrane or of the membrane module" will be understood to mean the temperature obtained within the membrane or the module by virtue of the entering gas temperature which passes through it, with from time to time the supplemental intervention of an external heating system of the membrane module or of temperature maintenance (thermostatic enclosure).
Thus, according to the case, to obtain the required level of performances, the entering gas will be heated by several dozens of degrees, or the gas will be maintained at ambient temperature, or else in certain cases this gas will be cooled below the ambient temperature, or even below 0.degree. C.
It will be recalled that, in the case of the production of nitrogen from air that the "output" of the membrane represents the proportion of nitrogen present in the entering mixture which is recovered at the outlet (residual) of the membrane, the O.sub.2 /N.sub.2 selectivity of the membrane represents as to itself the ratio of the permeances (frequently also called permeabilities) of the oxygen and the nitrogen through the membrane (sel.=Perm (O.sub.2) / Perm (N.sub.2)). The same type of reasoning is applicable for "hydrogen" membranes, given that here, the reasoning must be reversed in terms of extraction outlet because it is sought to recover the permeate mixture enriched in hydrogen.
Whether it is for the conventional air gases (nitrogen, oxygen) or the production of CO and hydrogen, the difficulties encountered are essentially of three types:
As to the first production problem of high purity gas, it has been proposed (see particularly U.S. Pat. No. 4,894,068, U.S. Pat. No. 4,119,417, or U.S. Pat. No. 5,240,471) to conduct the separation in multistage installations, which permit a reduction of investment and of energy consumed, relative to a single stage process. These patents teach that it is then advantageous to effect a recycling of the permeates (or of the residues according to the gas which is sought) downstream relative to the inlets of the membranes disposed upstream, or else the sweeping of the permeate from one upstream stage by the permeate of a downstream stage.
Thus, considering the example of the production of nitrogen from air, the recycling of the downstream permeate toward the inlet of the upstream membrane, this downstream permeate being enriched in nitrogen relative to the entering air, permits reducing the concentration of oxygen in the entering mixture, and hence increasing the recovery of nitrogen at the outlet.
Comparative evaluations made by the applicant show that these solutions, if they represent progress relative to conventional one-stage methods, are still not sufficiently competitive, particularly as to the cost of operation, for the case in which it is necessary to produce large size units, for which these solutions cannot compete with the other category of gas production on site which is production by preferential adsorption with pressure variation (often called PSA).
As to the problem described above of the untimely condensation of hydrocarbons in the membrane, the general attitude adopted is to limit voluntarily the hydrogen extraction output, by using for example "hydrogen" membranes at a different temperature from that at which they show the best output, which represents a "misfit" penalty.